Wind and other environmental factors, such as water-wave action and earthquakes, have prompted builders to construct man-made structures in such a way as to reduce the unnerving and potentially dangerous swaying of such structures. One approach to the reduction of environmentally-induced structural movement is the use of a tuned mass damper (TMD). A TMD dissipates vibrational energy into heat and prevents resonant vibration from reaching dangerous levels. Tuned mass dampers typically include a moving mass, a spring mechanism, and a dashpot mechanism. The spring and mass produce a natural frequency that is, by design, slightly less than the natural frequency of the vibration mode to be damped. The TMD interacts with the target mode of the base structure in such a way as to attract vibrational energy to itself where the energy is dissipated into heat by the dashpot element of the TMD. The dashpot, often called the “damper”, is a device that produces a force through it proportional to relative velocity across it. In a TMD for a building or other civil structure, the mass is typically a large concrete or steel body connected to the base structure through the spring and dashpot.
The Citicorp Building in New York City, built in 1977, was one of the first skyscrapers to use a tuned mass damper to reduce motion. The Taipei 101 Tower in Taipei, Taiwan employs a 730-ton, eighteen-foot diameter, steel sphere suspended as a pendulum and connected to the building through the pendulum fixture at the upper end of the TMD and massive hydraulic dampers at the lower end of the TMD. Its function is to reduce the building's response to environmental forces, including winds and earthquakes. This TMD reduces building sway by thirty to forty percent.
Although existing TMD systems provide some relief from structural sway, they typically employ hydraulic dampers, similar, to but larger than, automotive shock absorbers, as their dashpot elements. Hydraulic dampers use viscous oils that are under pressure and prone to leakage over the long service life required of civil structures. Organic, elastomeric, or liquid materials employed by such dampers may degrade over time. Viscosity of the oils used in hydraulic dampers is always sensitive to temperature. The resulting changes in viscosity make it difficult for the damper to operate in an optimal way over a typical range of outdoor temperatures.
Hydraulic cylinders, such as those employed by Taipei 101, must be quite long to accommodate the motion of the TMD mass, resulting in a large “footprint” for the TMD that may be difficult to accommodate within the base structure. Linear (translational) hydraulic dashpots require their own interface to the base structure in addition to the interface supporting the weight of the moving mass. This complicates the integration of the TMD into the base structure. Additionally, regular maintenance is required for the seals and fluids of a TMD using hydraulic dashpots.
TMDs for buildings are often in the form of a pendulum, with gravity combined with the pendulum length providing the spring mechanism. If the TMD mass is supported by wire ropes, these and their end fittings are subject to fatigue and may have limited life. Other approaches may include complex active components, which may add to the expense, increase the maintenance burden and reduce reliability.